Hi,
thats a difficult question. As far as I know most of the "würstchenbuden" (=sausage stands???) have their own recipe for Currywurst. As you can get a Currywurst on every corner, I never tried it myself.
I asked Mr. Google for "Currywurst Sauce" and got 100s of recipes. I went through some of them, and most of them were like the following, or similar to it:
1 bottle of ketchup (or do you call it tomato sauce in Britain?)
2 small onions
3 tablespoons Balsamico vinegar
2 tablespoons honey
2 tablespoons curry (powdwer)
some chili powder or cayenn pepper
Olive oil
adash of soja sauce
some tomato puree
water
steam the onions, add tomato puree and water
then adding ketchup, vinegar, honey. then boiling, but not too hot
then adding the rest, stiring, try it.
Good luck,
Christian